This invention relates to a method of approach in area cutting for cutting the interior of an area surrounded by the curve of an external shape. More particularly, the invention relates to a method of approach so adapted that in moving a tool toward a cutting starting point, the tool is moved obliquely with respect to a workpiece, so that the tool will cut into the workpiece without fail.
Forms of numerically controlled machining include cutting, in which the interior of an area bounded by the curve of an external shape is hollowed out down to a predetermined depth, and die milling in which the interior of an area is die milled. In such cutting of the interior of an area, as shown in FIG. 1, the process includes entering the curve OLC of an external shape of an area AR, cutting direction (direction of arrow A), cut-in direction (direction of arrow B), and cut-in pitch P; creating a cutting path PT.sub.i (i=1, 2, . . . ) on the basis of the entered data; performing cutting my moving a tool TL in the cutting direction along the created cutting path PT.sub.i ; creating the next cutting path PT.sub.i+1 by effecting a shift corresponding to the aforementioned pitch in the cut-in direction (direction of arrow B) after the completion of cutting along the above-mentioned cutting path; performing cutting by moving the tool in the cutting direction (direction of arrow A) along the next cutting path; and thereafter repeating this unidirectional cutting to cut the area AR. It should be noted that, for each cutting path PT.sub.i, two points P.sub.i, Q.sub.i where the curve OLC of the external shape is intersected by a straight line SL.sub.i determined by the cut-in direction and pitch are specified as machining starting and end points, respectively. A tool referred to as an end mill is used as the tool TL. As shown in FIG. 2, an end mill includes a bottom surface having cutting edges BT1, BT2, and a cutter side having a cutting edge BT3. Longitudinal cutting is performed by the cutting edges BT1, BT2, and transverse cutting is carried out by the cutting edge BT3. Little cutting force is applied in the longitudinal direction, and great cutting force is applied in the transverse direction.
The workpiece is a solid material prior to the cutting of an area. Moreover, the center position CP (see FIG. 2) of the bottom surface of tool (end mill) TL does not rotate (i.e., is stationary), even when the tool TL is rotated. Consequently, when the initial cut is to be made, even though the tool TL is moved for cutting feed from an approach starting point P.sub.a, which is located directly above the cutting starting point P.sub.i, as shown in FIG. 3, toward the cutting starting point P.sub.i while being rotated, the tool TL slides along the surface of the workpiece WK rather than cutting into the workpiece or, even if it does cut into the workpiece, it fails to do so smoothly and results in a machining error.
Accordingly, a hole is bored in advance at the initial cutting starting point P.sub.i so that the tool TL will be sure to cut into the workpiece WK when the approack is made. However, this method is disadvantageous in that it necessitates the hole boring step prior to the cutting of the area and prolongs machining time.